The present invention relates to an ultrasound diagnostic apparatus and an ultrasound image generating method, and particularly to an ultrasound diagnostic apparatus adapted to perform ultrasound transmission and reception using an arrayed organic piezoelectric elements.
In the medical field, ultrasound diagnostic apparatus employing ultrasound images have already been put to practical use. A typical ultrasound diagnostic apparatus for medical use transmits an ultrasonic beam from ultrasound transducers toward the inside of a subject, receives an ultrasonic echo from the subject on the ultrasound transducers, and electrically processes a reception signal corresponding to the received echo so as to generate an ultrasound image.
Recently, an increasing attention is captured by the harmonic imaging technology in which a harmonic component caused by the distortion in ultrasound waveform due to the non-linearity of a subject is received for imaging in order to conduct a more accurate diagnosis.
Such an ultrasound transducer unit as disclosed in JP 11-155863 A, for instance, is proposed as suitable for harmonic imaging, in which a plurality of inorganic piezoelectric elements having inorganic piezoelectric bodies of lead zirconate titanate (PZT) or the like and a plurality of organic piezoelectric elements having organic piezoelectric bodies of polyvinylidene fluoride (PVDF) or the like are stacked on each other.
An ultrasonic beam can be transmitted by the inorganic piezoelectric elements with high output and harmonic signals can be received by the organic piezoelectric elements with high sensitivity.
Between the inorganic piezoelectric elements and the organic piezoelectric elements, an acoustic matching layer is provided in order to efficiently deliver the ultrasound as emitted from the inorganic piezoelectric elements. The acoustic matching layer has a thickness meeting λ/4 resonance conditions with respect to the wavelength λ of a fundamental wave transmitted from the inorganic piezoelectric elements, which prevents reflection from the surface of the acoustic matching layer. In addition, the acoustic matching layer is formed of a material having an acoustic impedance of a medium value between the acoustic impedance values of the inorganic piezoelectric elements and of a subject as the living body, so as to carry out matching of acoustic impedances, that is to say, so as to cause ultrasound transmitted from the inorganic piezoelectric elements to efficiently enter the inside of the subject.
The inorganic piezoelectric elements and the subject, however, are quite different from each other in intrinsic acoustic impedance and, consequently, one acoustic matching layer may not be enough for an adequate matching effect.
It is possible indeed to use a plurality of acoustic matching layers with different acoustic impedances as stacked on one another in the order of acoustic impedance value, but the structure of an ultrasound transducer unit will be complicated.